GEOLOGY 105 Chapter 18: chapter 18 geology
28 Jan 2019
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Chapter 18
Introduction
● Geologists realized that erratics were extremely unsorted sediment (contained many different
sized clasts) and because of this they couldn't be typical stream alluvium because running water
sorts classes by size
● Many geologists attributed the erratics to have been moved by a flood that had been powerful
enough to spread a slurry of boulders, sand and mud across the continent, geologists thought this
until 1837 when Louis Agassiz proposed a different interpretation
● Louis Agassiz was a swiss geologist he came to the conclusion that erratics were actually moved
glaciers because ice was strong enough to carry rock and solid ice doesn't sort sediment by size as
it flows so once glaciers melted they left behind unsorted sediments
● Glaciers cover only about 10% of land on earth today but during the last ice age that ended less
than 12000 years ago as much as 30% of continental land surface was covered by an ice sheet ( a
layer of ice hundreds to thousands of kilometers across and as much as a few kilometers thick)
What is Ice
● Ice consists of solid water formed when liquid water cools below its freezing point
● Ice crystals are minerals that are naturally occuring with a definite chemical composition and a
regular crystal structure. They have a hexagonal shape so snowflakes grow into 6 pointed stars
● A layer of snow that has been compacted so grains stick together resembles a layer of
sedimentary rock
● Ice that appears on the surface of a pond is like igneous rock it forms when molten ice (liquid
water) solidifies
● Glacial ice is like metamorphic rock it develops when a mass of pre existing ice recrystallizes in
the solid state so it develops a new texture
How a Glacier Forms
● For a glacier to form the local climate must be cold enough that snow doesn't melt away
completely, so there must be enough snowfall that even in the summer a large amount of snow
can accumulate, and the surface on which it accumulates must have a gentle slope so that snow
falling on it does not slide away in avalanches and the accumulated snow must be protected from
the wind so it doesn't blow away
● Glaciers develop in polar regions because temperatures are cold enough so snow won't melt and
mountains because temperature decreases with elevation so at high elevations the average
temperature stays cold enough for snow to last
● Temperature of a region depends on its latitude and because of this the elevation where mountain
glaciers can grow and elevation down to which they can flow depend on latitude
● In present day climate glaciers at the equator don't descend lower than an elevation of 5 km but
glaciers formed at latitudes between 60 and 90 degrees can flow down to sea level
● Transformation from snow to glacial ice
○ Younger snow buries older snow
○ Freshly fallen snow consists of hexagonal crystals with sharp points and they don't fit
together tightly so fresh snow contains up to 90% air
○ With time points of snowflakes become blunt because they either melt into liquid or
evaporate into vapor
○ Once the flakes become blunt the snow packs together more tightly
○ As the snow becomes buried the weight of the snow on top increases the pressure
○ Under pressure the ice grains slide towards each other to fill pore space and the grains
slightly melt at their contact points which forms water that refreezes in pore space
○ As a result snow transforms into firn
○ As pressure continues to increase firn recrystallizes into a solid mass of new courser
interlocking ice crystals
○ This glacial ice tends to absorb red light which causes it to have a bluish color
○ Transformation of snow to blue glacial ice can take as little as 10s of years in regions
with a lot of snow and as long as thousands of years in areas with little snow
Categories of Glaciers
● Glaciers highlight coastal and mountain scenery in alaska, the cordillera of western north america
the alps of europe the southern alps of new zealand, himalayas of asia and the andes of south
america and they cover most of greenland and antarctica
● There are 2 main categories of glaciers on earth, mountain and continental glaciers
How do Glaciers Move
● In silicate rocks plastic deformation can only occur at depths in the crust where temperatures
exceed a few hundred degrees
● Bonds among water molecules in ice a pretty weak so plastic deformation in a glacier can happen
at depths of only about 60 m this depth is known as brittle plastic transition
● In polar glaciers where ice remains below its melting temperature plastic deformation takes place
in the solid state
● In temperature glaciers where ice is at or near its melting temperature plastic deformation
involves grains sliding past their neighbors on thin water films
● Above the brittle plastic transition glacial ice tends to be too breakable to flow easily so as the
deeper parts of a glacier flow the shallower parts may crack
● Glaciers move because the force of gravity can overcome the strength of ice
● Glaciers flow in the direction in which its top surface slopes because of this glaciers can move up
and over ridges or hills in the layer that underlies their base
● Glaciers flow at rates of 10-300 m per year
● Not all parts of a glacier move at the same rate
○ Interior of a valley glacier moves faster than its margins
○ Top of a glacier moves faster than its base
● The process of basal sliding takes place when glaciers flow into conditions where water can
accumulate and mix with sediment at the base of a glacier
● During basal sliding the glacier glides along on a wet sediment slurry without coming into
frictional contact with bedrock and as a consequence may undergo a surge which means that it the
glacier moves must faster than normal
Glacial Advance and Retreat
● Snowfall adds to the glacier in the zone of accumulation whereas ablation subtracts in the zone of
ablation the boundary between these 2 zones is the equilibrium line
● If the rate of ice build up in the zone of accumulation exceeds the rate at which ablation occurs
below the equilibrium line then the terminus or the toe (downslope end of the glacier) moves
forward in previously unglaciated regions
● If the rate of ablation below the equilibrium line equals the rate of accumulation then the position
of the toe remains fixed even though ice continues to flow towards the toe
● If rate of ablation exceeds the rate of accumulation then the position of the toe moves back
towards the origin of the glacier
● When a glacier retreats only the position of the toe moves back towards the origin for ice always
flows towards the toe
● Under the zone of accumulation ice flows down towards the base of the glacier as new ice
accumulates above it
● Under the zone of ablation ice gradually moves up towards the surface of the glacier as overlying
ice ablates
● Ice following curving flow paths for this reason rocks picked up by ice at the base of the glacier
may eventually get to the top of the glacier
Ice in the Sea
● When valley glaciers enter the sea they become ice tongues- a long narrow sheet of ice projecting
out from the coastline
● When continental glaciers enter the sea they become broad flat sheets known as ice shelves
● In shallow water glacial ice remains grounded but in deeper water ice floats so that about 88% of
its mass is under water
● At the toe of a tidewater glacier blocks of ice calve off and tumble into the water with an
impressive splash, a free floating chunk that rises 6 m above the water and is at least 15m long is
an iceberg
● The earth's north polar ice cap consists of sea ice formed on the surface of the arctic ocean
● Some sea ice floats freely but some protrudes outward from the shore
The Process of Glacial Erosion
● Glacial incorporation is when ice surrounds loose rock and carries it away
● Glacial plucking is when a glacier breaks off fragments of bedrock, plucking occurs when ice
flows into joints that intersect the bedrocks surface
● As glaciers flow sand and silt embedded in the ice act like the teeth of a giant rasp and grind away
the substrate, this process of glacial abrasion pulverizes rock into a fine powder known as rock
flour and yields shiny glacially polished surfaces on bedrock
● Striations run parallel to the flow direction of ice
● The freezing and thawing of a mountain glacier help fracture the rock bordering the ice edge high
in the mountains once the rock has been fractured it falls on the ice or gets picked up at the base
of the ice and moves downslope with the glacier as a result a bowl shaped depression (cirque)
develops of the side of the mountain
● If the ice later melts than a tarn develops at the base of the cirque
● Glacial erosion drastically modifies the shape of a valley
● A profile across a river in unglaciated mountains typically flows down a V shaped valley
○ The V develops because river erosion occurs only in the channel and mass wasting causes
the valley slopes to approach the angle of rest
● A profile across a glacially eroded valley resembles a U
● A U shaped valley forms because the combined processes of glacial abrasion and plucking not
only lower the floor of the valley but also bevel its sides
